1. Field of the Invention
The present invention is generally related to optical communication systems, and more particularly to an optical communication system utilizing a single polarized, phase modulated transmitted beam that provides substantially linear recovery of an analog communication signal.
2. Description of the Prior Art
Analog optical communication links are known in the prior art. Conventional optical analog links employ intensity modulation techniques to convey the analog information on an optical beam of light. Such analog optical links are utilized by the cable television industry to transmit video images using the conventional RF analog modulation format for television video. Intensity detection at the receiver using conventional photodetectors enables the light intensity to be linearly converted to an analog voltage corresponding to the signal that is to be transmitted by the link. However, inherent to these analog intensity modulation optical links is an acceptance of a non linearity associated with the intensity modulators used in the transmitter. Mach-Zehnder intensity modulators, which are commonly employed in optical intensity modulation analog links, have a non linear transfer function that yields a sinusoidal intensity variation with a linearly changing applied analog modulation voltage. Similarly, electro absorption modulators also yield a non linear intensity variation to a linearly applied analog modulation voltage.
This inherent non linearity associated with intensity modulators has led to a consideration of using optical phase modulation in the transmitter as an alternative to intensity modulation. Optical phase modulators that can achieve a linear change in the state of the optical phase with a linearly changing analog modulation voltage are known in the art. Modulators can be made from electro optic materials that change their refractive index linearly with applied electric field supplied by a linearly changing analog modulation voltage. The linearly changing refractive index causes the optical path length through the modulator to linearly change. This linearly changing optical path length causes a linearly changing state of optical phase corresponding with a linearly changing analog modulation voltage. Thus, an optical phase modulator can be used in the transmitter to deliver a linearly varying optical signal in contrast to the inherent non linearity associated with intensity modulators.
The utilization of a linear phase modulator in an analog optical communication link requires that the state of optical phase be detected at the receiver. Conventional approaches for this utilize optical interference techniques that cause the phase varying light to become detectable with photodetectors as intensity variations. A common approach used for optical phase state detection is to interfere the phase modulated communication light with an unmodulated reference beam of light that has been split from the initial light source prior to applying the phase modulation. The process of utilizing optical interference techniques to detect the state of optical phase leads to a non linear sinusoidal intensity variation that corresponds to the linearly varying state of optical phase. Thus, this conventional phase detection process leads to a non linearity in the detected analog signal. This non linearity inherent in the conventional phase detection process negates the linearity achieved by the phase modulator and results in an analog optical communication link that is as non linear as the conventional intensity modulation analog optical link. Thus, all analog optical communication links are degraded in performance by an inherent non linearity that distorts the original analog signal that is to be conveyed.
What is needed, therefore, is an analog optical communications system that is capable of detecting the state of opticalphase of a phase modulated communication signal in a way so as to produce an analog voltage signal that is linearly related to the state of optical phase of the phase modulated optical signal. Such an analog optical communications system thus will be capable of conveying an analog signal without any non linear distortion.
The preceding and other shortcomings of the prior art are addressed and overcome by the present invention which provides generally an optical communication system for communicating an analog communication signal.
Briefly, the optical communication system comprises an optical transmitter and an optical receiver separated by an analog optical link. The transmitter comprises means for generating a polarized communication light beam that includes an orthogonally polarized reference light beam and phase modulated communication beam that is generated by means responsive to the analog communication signal.
The receiver comprises means for separating the orthogonally polarized reference light beam and the phase modulated communication light beam from the polarized communication light beam means responsive to the reference light beam and an RF signal and operative to develop a shifted light beam that is shifted in frequency by the RF frequency, means responsive to the phase modulated communication light beam and the shifted light beam and operative to interferometrically combine the communication light beam with the shifted light beam to provide a heterodyne signal including information regarding the state of phase of the communication light beam means responsive to the heterodyne signal and operative to produce an electrical signal at the optical beat frequency corresponding to the RF and with a phase corresponding to the state of phase of the communication light beam, and means responsive to the RF signal and the electrical signal and operative to Provide an output signal that has a linear correspondence to the state of optical phase said communication light beam and the analog communication signal.
More particularly, the detection of the relative phase of means for converting the RF signal into a first digital waveform, a first digital divider for dividing the first digital waveform by a predetermined integer to form a first square wave, means for converting the heterodyne signal into a second digital waveform, a second digital divider for dividing the second digital waveform by the same predetermined integer to form a second square wave, an exclusive OR circuit for processing the first and the second divided square waves to form a pulse waveform, and means for integrating the processed pulsed waveform to provide the output signal having a magnitude that varies linearly relative to the state of optical phases of the communication light beam and the analog communication signal.
Other aspects of the invention separately describe the transmitter and the receiver.
The foregoing and additional features and advantages of this invention will become apparent from the detailed description and accompanying drawing figures below. In the figures and the written description, numerals indicate the various elements of the invention, like numerals referring to like elements throughout both the drawing figures and the written description.